Natural gas and other gaseous fuels such as propane are considered by many to be superior to other fuels such as diesel fuel and the like for engines because the gaseous fuels are generally less expensive to operate, and produce significantly lower emissions. The overall performance of an engine in terms of combustion efficiency, speed control, and exhaust emissions, greatly depends on controlling the mixing of the air and fuel into an appropriate ratio for combustion and on regulating the flow of this mixture into the combustion chamber or cylinder of the engine. Precise and reliable control of the combustion is very important for the efficiency and the safety of the combustion process. For example, it is well known that excess fuel yields fast combustion rates and high combustion temperatures, which result in high nitrogen oxide (NOx) emissions. Too little fuel can cause incomplete combustion, which results in high unburned hydrocarbon (UHC) emissions and loss of power.
One drawback of gaseous fuels is that they require an ignition source to begin the combustion process. Spark plugs are typically used to ignite the fuel in gaseous fueled engine combustion chambers. However, the use of spark plugs decreases the maintenance interval of the engine, which in turn leads to increased operating costs.
Several engine manufacturers have introduced, or are developing, micro pilot ignited natural gas engines. The advantages claimed and substantiated by test data include it being an adjustable high energy ignition system and having good combustion stability, good thermal efficiency, power densities equal to the diesel engine, and NOx levels about one tenth that of the diesel engine. A small pilot fuel injector, typically using diesel fuel, fuel oil, or even engine oil as the pilot fuel, is used and replaces the spark plug in the open chamber (or prechamber) of natural gas engines. Limited amounts of pilot fuel are injected into the combustion chamber (or pre-combustion chamber) to initiate combustion by compression ignition. The pilot fuel ignites and bums at a high enough temperature to ignite the gaseous fuel charge in the combustion chamber. The added complexity of having two fuel systems (e.g., pilot fuel and natural gas) is compensated for by lower operating costs. Because power density was limited by spark plug life, increases in power density, fuel efficiency, and increased time between scheduled maintenance are possible due to elimination of the spark plug as the combustion ignition device.
It is well known that the ignition characteristics of gaseous fuels vary with engine operating conditions. For instance, the air/fuel ratio and density in the combustion chamber, changing as a result of changes in engine load, affect the combustion characteristics of the charge in the combustion chamber. The optimum amount of pilot fuel required to successfully ignite the gas charge can vary depending on the engine combustion chamber design, the fuel temperature, the fuel quality, the air temperature, the air/fuel ratio in the combustion chamber, the residual exhaust in the chamber, the pilot fuel cetane number, and the pilot fuel spray characteristics. One approach to inject the pilot fuel is to always inject sufficient pilot fuel to assure adequate primary fuel charge ignition under all conditions. However, this approach is undesirable because the pilot fuel combustion produces higher NOx emissions (too fast combustion) and higher fuel consumption (rich fuel mixture). Too little pilot fuel injected can result in the mixture not igniting, which will cause a drop in horsepower output and an increase in unburned hydrocarbon (UHC) emissions.